Eh, I can't really say anything and not be considered biased here.
It does not really work to Rin's credit, that Arthur is an expert demon slayer, who we have yet to even see go full-out. He has years of experience in both live-combat and demon-sword use ahead of him, not to mention we saw him take down Shura (another demon-sword user) rather easily.
So yeah, this one is clearly in Arthur's favour. He can beat Rin as he is now, and can probably beat him easily when he is in 'berserk' non-thinking demon mode. This stuff is kinda in his job description.